A wide range of analog circuits are provided with voltage or current references in the form of DC quantities. Such DC quantities depend barely on power supply or process parameters, but usually have a predefined relationship with temperature. Such references are created in order to produce a DC voltage or current that is independent from power supply or process parameters but are associated with a predetermined temperature characteristic. In most applications, the desired temperature characteristic may be: 1) proportional to absolute temperature (PTAT), 2) a constant Gm characteristic, i.e. a transconductance constant of some transistors, or 3) independent from temperature. To implement a source of reference voltage, concerns are mainly involved in the control over temperature and power supply in order to achieve a predetermined relationship to temperature and a substantial independence from the power supply. As semiconductors almost have no temperature-independent parameters, the independence from temperature has to be achieved by appropriate combinations of selected power-independent parameters with positive and negative temperature coefficients. Moreover, these selected parameters shall be independent from power supply. A band-gap of a semiconductor is defined as the difference between the bottom of its conduction band and the top of its valence band. A band-gap voltage reference (also briefly known as band-gap) provides a temperature-independent voltage reference generated from a sum of a voltage proportional to temperature and a voltage drop across a diode, with temperature coefficients of them cancelling out. As this voltage reference is comparable to the band-gap voltage of silicon, it is also known as band-gap reference. Some conventional band-gap architectures may also adopt an output voltage different from the above band-gap voltage.
It is noted that in existing band-gap reference circuits, a relatively low power supply voltage may result in an inaccurate output voltage of the band-gap reference circuit.